From July 14 to 16, 2000, the surface of
the sun exploded. Huge, bright flares spewed out into space like
powerful fountains colorfully lit from beneath. Within a few hours, the
solar storm bombarded Earth with a shower of positively-charged hydrogen
atoms, called protons, causing scientific and communications satellites
to short-circuit. Through a series of chemical reactions in our
atmosphere, the protons drastically diminished the upper-most areas of
the ozone layer, a protective blanket mostly in the stratosphere that
blocks life-threatening ultraviolet radiation from reaching the Earth.
This shower of protons, known by solar science insiders as the Bastille
Day event, was the third largest of its kind in the last 30
years.

Atmospheric scientist Charles Jackman
and a team of researchers from NASAs Goddard Space Flight Center
and Hampton University in Virginia recognized a rare opportunity to
gather further proof that solar storms destroy ozone. They already knew
that when protons bombard the upper atmosphere, they break up molecules
of gases like nitrogen and water vapor. Once freed, those products
readily react with ozone molecules and reduce the ozone layer. So,
Jackman and his colleagues recalled specific Northern Hemisphere
atmospheric data from NASA and National Oceanic and Atmospheric
Administration (NOAA) satellites that continuously monitor the
composition of gases and molecules that surround our planet.

Their findings, published in the August 1, 2001, issue of Geophysical
Research Letters, show that less than one percent of total atmospheric
ozone in the Northern Hemisphere can be quickly reduced by one of these
events. It is an indication of the power of the sun to actually
affect the atmosphere in a sudden, cataclysmic way, Jackman says.
While the results do not show a significant impact on human health,
especially considering that most of the ozone loss documented in this
study occurs over the northern polar region, they are important
scientifically. The study gives detailed and quantified knowledge of how
a solar storm affects upper-level ozone. As scientists race to better
understand humankinds role in ozone loss, they must first be able
to tease out the natural causes.

On July 14th, 2000, an
active region of the sun (called AR9077) produced a massive flare. The
event also blasted an enormous cloud of positive-charged particles
toward planet Earth, triggering magnetic storms and dramatic auroral
displays. This striking close-up of AR9077 was made by the orbiting
Transition Region and Coronal Explorer (TRACE) satellite shortly after
the flare erupted. Suspended in an arcade of magnetic loops, the image
shows a one million degree hot solar plasma cooling down. Plasma is a
gas that has been heated to a state where it contains ions and
free-floating electrons. The false-color image covers an expansive
230,000 by 77,000 kilometer area on the Suns surface (Earths
diameter is about 12,800 kilometers) and was recorded in extreme
ultraviolet light. Collectively resembling a popular slinky
toy, the enormous loops are actually magnetic field lines which trap the
glowing, cooling plasma above the relatively dark solar surface. After
the flare, AR9077s activity decayed as it was carried farther
across the Earth-facing hemisphere of the Sun by solar rotation. Active
regions like AR9077 appear as groups of dark sunspots in visible light.
(Image courtesy TRACE)

A lot of impacts on ozone, like those caused by humans, are
very subtle and happen over long periods of time, says Jackman.
But when these solar proton events occur you can see immediately a
change in the atmosphere.

The map at left shows
the distribution of ozone in the atmosphere on July 14th and 15th, 2000,
measured by the Total Ozone Mapping Spectrometer (TOMS). (Image courtesy TOMS science
team)

Jackman explains that ultraviolet
radiation from the sun continuously strikes the upper atmosphere. These
harmful ultraviolet rays would make life on Earth impossible if it
werent for the ozone layer that absorbs the radiation. Ozone forms
when three atoms of oxygen bind together to create a single molecule
(O3), but when ozone absorbs
ultraviolet radiation, the molecules split apart into a single free
oxygen atom, and an oxygen molecule of two tightly bound oxygen atoms
(O2). O2 is the oxygen we breathe, and it makes up about
21 percent of the Earths air.

The free oxygen atom is so reactive and there is so much
O2 around, that out of 1001 times that
this reaction occurs, 1000 times it will reform, Jackman says. As
a result, day-to-day break down of ozone by ultraviolet radiation
doesnt affect the overall amount of it in the atmosphere.
The only way to destroy ozone is for that free atom of oxygen to
reform with something else, Jackman says.

Thats where solar proton events enter the picture. When protons
from the sun hit the atmosphere they break apart both water vapor and
nitrogen gas, which accounts for 78 percent of our atmosphere. The
nitrogen gas molecules (N2) disconnect
and leave two free nitrogen atoms. Nitrogen atoms are highly reactive
with O2, creating oxides of nitrogen.
Once formed, these molecules can last for weeks to months depending on
where they end up in the atmosphere before they get destroyed. Protons
also break up water vapor (H20) into a
hydroxide molecule (OH) and a free-floating single atom of hydrogen.
Both of these products also react easily with ozone and reduce its
levels in the atmosphere. Fortunately, oxides of hydrogen are
short-lived and only stay together as long as the rain of protons keeps
coming.

For more information
about the formation and destruction of ozone, and the structure of the
atmosphere, see Ozone in the Stratosphere.

Jackman says that knowledge of solar
protons entering the atmosphere and disrupting ozone production is not
anything new. We knew that these protons were streaming into the
atmosphere, he says. The prediction from a few decades ago
was that these protons would create free nitrogen atoms. They knew that
from lab experiments.

Scientists like Jackman regularly try to improve their computer
models to more accurately simulate the atmosphere, and, in this case, to
better determine present ozone levels and to predict future amounts. In
order to do that, they must account for daily, short-term and long-term
variations caused by different factors. Solar proton events help
us test our models, Jackman says. Thats because they offer
quick, verifiable results that can then be used to test model-based
predictions.

Jackman says that prior to his study scientists had good measurements
of ozone levels during solar storms, but they didnt have as
comprehensive a measurement for the effect that protons have on nitrogen
oxides (NO and NO2). This study
documents actual levels of destruction, by taking readings of ozone and
nitrogen oxide levels before and during the event. Every ninety
minutes, we had a new measurement of nitrogen oxides, so we could watch
them develop as the solar proton event developed, Jackman said.
Scientifically speaking, the change in the amounts of nitrogen oxides
and ozone was huge in the uppermost regions where ozone lies. And those
were the areas where the computer models had some inaccuracies, Jackman
said. Most of the damage is done near the top of the atmosphere because
it takes a lot of energy for protons to penetrate to lower atmospheric
regions. Fortunately for us, much less ozone exists at these
heights.

Based on his prior experience with protons and ozone, when the solar
storm broke on July 14 and Jackman saw the huge proton fluxes measured
by another NOAA satellite, he says that he knew right away this was a
unique opportunity to gather data. The big solar storms dont
really occur that often, and this event was off the charts, he
says. So we knew if we looked wed have
something.

The sudden onset of a
high energy proton blast from a solar flare is captured by the Large
Angle Spectrometric Coronagraph (LASCO) C3 instrument on board the Solar
and Heliospheric Observatory (SOHO) on July 14, 2000. The particles
reached the spacecraft in less than one half hour and continued pelting
it for days. The snow-like white spots are the protons hitting the
spacecrafts imager. Solar flares are explosions on the Sun that
happen when energy stored in twisted magnetic fields (usually above
sunspots) is suddenly released. (Image courtesy of the SOHO/LASCO
consortium.)

The atmospheres layers include the
troposphere, the stratosphere, the mesosphere and the thermosphere. The
troposphere lies closest to Earth at zero to nine miles up, and the
thermosphere floats furthest away at about 60 miles up. Ozone is
distributed over these layers. About 90 percent lies within the
stratosphere, and most of that amount-over 80 percent of the total
ozone-stays in the middle and lower stratospheric regions. Around 9
percent can be found in the troposphere, and the remaining one percent
sits in the mesosphere.

By observing the Bastille Day solar event, Jackman and his colleagues
found that the short-term effects of hydrogen oxides destroyed up to 70
percent of the ozone in the middle mesosphere. The mesosphere was
really shaken, Jackman says. At the same time, ozone loss caused
by longer-term nitrogen oxides cut out close to nine percent of the
ozone in the upper stratosphere. But, Jackman says, only a few percent
of total ozone resides in the mesosphere and upper stratosphere.

If you look at the total atmospheric column, from your head on
up to the top of the atmosphere, this solar proton event depleted less
than one percent of the total ozone in the Northern Hemisphere,
Jackman said. While that doesnt sound like a lot, scientifically
speaking the numbers for the specific atmospheric regions are quite
significant.

This is an instance where we have a huge natural
variance, Jackman says. The ultimate goal of a lot of our
work is to understand the human impacts on ozone. In order to do that,
you have to first be able to separate the natural effects on
ozone.

The image sequence and
graph above show the arrival of massive numbers of protons from the
Bastille day solar flare. The top row of images show the crescent Earth
on July 14, 2000, from the Visible Imaging System aboard the Polar
spacecraft. The first image (left) was taken at 10:33 UTC, each
succesive image was acquired at 7 to 8 minute intervals, with the last
image taken at 11:32 UTC. The oncoming wave of protons hitting the
sensor caused the increasing amount of noise (white dots) in the image
sequence. The graph below shows proton flux from the same time period,
measured by the High Energy Proton and Alpha Detector aboard GOES 8 (a
NOAA weather satellite). The density of high-energy protons rose from
less than 1 protons per square centimeter per second per steradian to
over 9. This was just the beginningon July 15 over 800 protons
were counted per second. (Images courtesy International Solar-Terrestrial Physics, graph by
Robert Simmon, based on data from the Space Physics Interactive Data
Resource)